Thursday, November 1, 2012

Nikon D600 autofocus with fast subjects in low light

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In response to a comment on the post about the Nikon D600 autofocus, I re-examined the Nikon D600's autofocus performance.

In DigitalRev's comparison between the D600, the D800 and 5DIII, Kai found that the D600's autofocus was frustrating under the conditions he tested. I found that observation surprising, because I've never gotten an impression that the D600's AF was hesitant.  On the contrary, I found that it was able to focus in low light (as low as EV 1) with no issues, and was able to capture fast-moving subjects as well.
Anyway I took some test shots under conditions similar to Kai's to see if the D600 would hesitate.  In Kai's test, he was taking photos of boxers.  The gym had artficial lighting overhead that seemed dim, and a large window that did not appear to be facing the sun.  Kai was using the D600 with the 28-70 2.8G.
Our living room seemed to have the same conditions.  There are patio doors that are similar to a large window, and I turned off lights and just used the TV for artifical lighting.  I took the shots while it was overcast outside, so it was dim.  The light level was around 4 EV (ISO 16,000, f/2.8, 1/320).  I asked my son to strike some poses in his Halloween costume (Chop Chop from Skylanders).  I used the 24-70, focus mode was AF-C, for focus area I tried Auto and single, and I was using focus priority instead of release priority.
Under these conditions, I did observe a delay when I pressed down the shutter.  I would hold the shutter down and the camera would hunt for a second or so before locking focus and releasing the shutter.
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That was the case whether I used Auto Area AF or single AF point.  I don't know yet for sure why there was a delay under those circumstances, and I didn't have time to pin the reason down at that time.
The strange thing is, I continued to use the 24-70 during the rest of the day and took some more shots, but I didn't notice any autofocus hesitation.  I don't think it's because of the mix of artificial lighting and natural lighting.  This following shot had somewhat similar lighting conditions.  The room had artificial lighting overhead, there were a couple of doors that were open and were acting like large windows.  The kids were swinging their swords wildly.  The only difference was that it was brighter (ISO 560, f/4, 1/250, +1.5 EV in posprocessing = around 8.2 EV).
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Later in the day, I took my son to Disneyland and California Adventure (our daughter was sick so my wife stayed home with her).  Here is a shot where we were on one of the rides at California Adventure, and we were veering left and right, and the light was dim:
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The light was around 2.3 EV (ISO 25,600, f/4, 1/80).  No hesitation.
Here is a shot where it was dim, and the subject (my nephew) was playing around.  I did not note any hesitation either.
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The light levels in that shot were quite low - here is a shot under the same light conditions but without flash showing an EV of 4 (ISO 25,600, f/2.8, 1/500).  (BTW I did not notice any delay for the shot below either).
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I don't know yet the circumstances when there is a delay with the D600.  Low light and an active subject alone are not enough - sometimes.  My hypothesis is that there was no delay in the shots later in the day because I was half-pressing the shutter.  I know that when I go from not pressing the shutter to suddenly fully pressing the shutter and I'm using a wide aperture lens at a close distance to the subject, in low light and with a moving subject, then the camera doesn't take the shot right away (note: I use focus priority).  To be honest, though, I don't think any of my other cameras do that either (I don't recall my D3 being able to do that).  Anyway I will look into this issue further and try to pin it down.

11/6/12 update:  I tried the autofocus in very low light (25600 ISO, f/2.8, 1/60 = EV 1), close to the subject (i.e. shallower DOF).  With the center AF point, the D600 is able to focus without a problem.  With the outer focus points, it sometimes focuses quickly but sometimes hunts (even when I half-press).  Therefore at least one workaround when the D600 hunts in low light is to use the center AF point.

11/5/12 UPDATE:
My co-author MShafik was kind enough to run a test with his Canon 5DIII to see if there would be any delay from autofocusing in low light and shallow depth of field.  Here are his findings:

I did the experiment with the 100 f/2.8 L, the 50 1.4 does not have true USM and thus a bit slower to focus. I switched the AF function to the shutter button (I usually use BBF), switched the camera to manual mode and tried the following scenarios, in each test I focused at a different distance too far from my subject, then focused on my subject, in all scenarios the camera was set to focus priority.

Low Light 1: Exposure: f/2.8, shutter 1/100, ISO 25,600

One shot AF (AF-S in Nikon) - Single Point: fully pressing the shutter, the camera waited until the lens moved to the required distance and fired away, trying to shoot subjects at close distances was immediate, here I was limited with the lens autofocus speed since I was quite far initially. I bet if I was using the 85 1.8 there would be no delay, it is a speed demon.

One shot AF - Auto Area: exact same result, low light has nothing to do with the focus speed, the same would have happened in day light.

AI Servo AF (AF-C in Nikon) - Single Point & Auto Area: Initial focus from one end to the other took less delay, but the subsequent shots (keeping the shutter pressed and moving the lens to different targets) were literally instantaneous, I always have my camera on AI Servo, always.

Low Light 2: Exposure: f/2.8, shutter 1/20, ISO 25,600

Exact same results as above, but with a slightly longer delay in one shot mode.

Conclusion for Canon 5DIII: you could have instantaneous initial focus if the lens didn't have to move a long distance (from one end to the other), or you had a seriously fast focusing lens.

11/8/12 UPDATE: Helpful D600 autofocus tips here.